Flashii.net Tekkit Classic Server
It's more trouble than it's worth!
If you've already done all this and are just looking for the server IP, here you go;
tekkit.flashii.net. You can change your player skin here.
I've decided to spin up a Tekkit server again, this time hosted in NYC so overall latency should be better for most people. I previously stated that I'd be making my own launcher that uses Flashii IDs instead of Mojang accounts, but that ended up being more trouble that it's worth. The version of Tekkit we use is Tekkit Classic 3.1.2, this version is based on Minecraft 1.2.5 from 2012. This wouldn't be that important of a detail if it weren't for numerous things being kinda broken. Details on how to patch it up are detailed in the rest of this post!
Despite all the things you have to do to get the game going, I'd still prefer to keep it one account per user to keep random people out as well. You can automatically whitelist yourself using the webpage listed below. Keep in mind that the whitelist might take a short while to refresh on the Tekkit server's end.
In order to install the game you must download the Technic Launcher from technicpack.net, you can put this file anywhere you want.
After starting and logging in go to the "MODPACKS" tab at the top left. Type "tekkit classic" in the field where it says "Add Pack or Search". Tekkit Classic should be the first result, click it and ensure it says "Official Technic Modpack" to ensure you have the correct one.
Click "Install" in the bottom right. Make sure you have Tekkit Classic 3.1.2!
I've made a utility to automate the next two steps you can download it here for Windows and the source code is available here. Huge filesize in comparison to the actual amount of code, but that's because the executables contain the entire .net framework.
Once you've landed on the server you might notice that no one has a skin, or those that do have skins won't be able to change it. This happens because Minecraft stopped updating their old skin server. To mitigate this I've made a minor patch which updates the skin urls in minecraft.jar to ones maintained on Flashii.
In order to install the patch you must close the game, go to the "Modpack Options" in the launcher, click "Open" to go the modpack's folder and then enter the "bin" folder.
You must now open "minecraft.jar" as an archive, you can easily do this with 7-zip or WinRAR. Personally I use 7-zip which adds an easily accessible context menu entry to open the file as an archive, it's been a while since I've used WinRAR but I'm pretty sure it's a similar process.
Once you've opened minecraft.jar you want to drag the files in skinpatch.zip into it. This will overwrite the files containing the Amazon skin urls with the Flashii ones.
Another thing you might notice is that a lot of sounds are missing. This is because the old Minecraft asset server is dead. By dumb luck I found an old copy I had of the game which had all these files still downloaded.
In order to make these files available to the game you must close the game, go to the "Modpack Options" in the launcher and click "Open" to go the modpack's folder. Once you're there extract the contents of resources.zip to it. Once it's done copying you should hear the missing sounds again in the game!
Epic easy way to install this shit lmkoa
Step 1: Install MultiMC
Step 2: Go to MutliMC install folder and then go into the "instances" folder
Step 3: Extract this into the instances folder
Step 4: Restart MultiMC and you should see "Tekkit Flashii" in MultiMC
Step 5: Double click and play etc